Blood leucocytes from humans and rabbits release, when properly stimulated, a pyrogenic protein. This leucocytic pyrogen causes experimental fevers in animals and man and perhaps also the fevers associated with a number of human diseases. We have found recently that liver and spleen cells in response to Poly I:Poly C are also capable of releasing a pyrogen. We plan to determine the type of cells responsible for producing this new tissue pyrogen and to study its chemical and biological properties. In addition, we hope to compare the mode of production of leucocytic pyrogen with that of tissue pyrogen.